In order to adapt to the rapid growth of mobile data traffic, the deployment of small base stations is the trend of wireless communication development. Small base station facilitates managing and using existing wireless spectrum resources more efficiently. As compared with the macro base station, the small base station has smaller coverage, higher space utilization of the spectrum, and more flexible layout. There may be a number of small cells served by small base stations within the coverage of each macro base station in the future.
With the increase of the number of small base stations, in the coverage of a specific small base station, there may be not only multiple served users, but also multiple interference users which are not served by the small base station. Interference users may be the users served by other small base stations, or may be the users served by a macro base station. If all users within the coverage of a small base station are divided into a served user set and an interference user set, each set may include multiple users. The interference user set is a set of which signals need to be suppressed, while the served user set is a set of which signals are to be maintained. It is desired to implement multi-user communication without mutual interferences in the served user set. Therefore, interference coordination between small base stations and between the small base station and the macro base station has extremely important impact on the management of the entire wireless network.
Interference coordination technology mainly aims to minimize the interference between neighboring cells by coordinating different base stations to use different communication resources. The existing technology mainly considers the following three aspects: frequency domain, time domain and power control. From the frequency domain perspective, the cells that are adjacent to each other use spectral resources orthogonal to one another, and different frequency sets are allocated to the user located in the center of the cell and the user located at the edge of the cell, thereby improving the communication performance of the use located at the edge of the cell, or orthogonal subcarrier resources are reasonably reused on the basis of the orthogonal modulation technique. From the time domain perspective, sub-frame management is mainly performed, such that the interference users have limited load or even no load in some sub-frames, thereby ensuring the reliable communication of the served users in these sub-frames. The power control technology improves the communication quality by sacrificing a part of the throughput, for example by reducing the transmission power of the interference base station to ensure the communication performance of the interfered user. These three aspects of the technology facilitate reducing the interference, but to some extent at the expense of other aspects of the system performance.
On the other hand, the smart antenna system is dedicated to the development of space resources. By deploying the antenna array, the pattern of the array can be changed by adjusting weighting coefficients of respective elements so that the beam always points to the DOA (Direction of Arrival) of the user's signal, with the zero point being aligned with the interference signal, such that the beam automatically follows the user's signal. This technology can improve the gain and signal-to-noise ratio of the antenna, such that the number of the users can be increased under the condition of same frequency domain, same time slot and same code. It is an effective way to solve the shortage of resources in current frequency domain and to improve the communication capacity. The conventional arrival angle estimation techniques include subspace-based arrival angle estimation algorithms. Typical algorithms may include, for example, MUSIC and ESPRIT. However, these algorithms can estimate only the arrival angle of all signals arriving at the antenna array, but cannot obtain the arrival angle corresponding to the signal of each specific user terminal. Therefore, the interference to other user terminals cannot be avoided when the base station performs downlink transmission.
Therefore, the future wireless communication system in which a large number of small base stations are deployed puts forward the requirements of interference coordination and multi-user communication in a multi-cell multi-user multi-interference environment.